A large family of materials known as thermoplastics are rigid and brittle at ambient temperature but soften and flexible when heated. Thermoplastics are often divided into either of two groups:

  • polymerized thermoplastics
  • Thermoplastics that are semi-crystalline

The main difference between the two is that although amorphous plastics go from soft to hard states more gradually, semi-crystalline polymers crystallize (become hard) at a certain temperature. Materials like nylon and polypropylene, which are often used, are examples of semi-crystalline thermoplastics. Some of the most well-known amorphous thermoplastics are acrylic, polystyrene, and polycarbonate.

Amorphous and semi-crystalline polymers.








Polyamide 6,6


Polyamide 11


Polyphenylene sulphide

Polyamide (amorphous)

Polyethylene terephthalate





Acrylonitrile butadiene styrene

High Density Polyethylene


Low Density Polyethylene

Characteristics Of Amorphous Thermoplastics

Since amorphous thermoplastics are simple to thermoform, they make a great option for the majority of injection molding applications. Amorphous thermoplastics are typically translucent in their solid state, in contrast to semi-crystalline plastics which tend to be opaque. They are frequently utilized in optical products like ski and swimming goggles as a result.

In order to be processed into the desired shape, plastics must contain a large number of macromolecules that are created under the influence of pressure and heat. Amorphous thermoplastics and semi-crystalline thermoplastics are the two main categories of thermoplastics. Thermoplastics are categorized as amorphous thermoplastics when they are formed at a temperature higher than the glass transition temperature Tg. While semi-crystalline thermoplastics are categorized when the temperature is above the melting point (Tm).

In order to maximize the cost-saving benefits offered by thermoplastic injection molding technology, precise material utilization and cycle times must be followed. When compared to semi-crystalline materials, amorphous thermoplastics have less tendency to buckle under pressure and mold shrinkage. Once they reach the glass transition temperature Tg, they quickly lose their strength and rigidity. Amorphous thermoplastics soften progressively as the temperature rises because to the randomly organized molecular structure.

Are thermoplastics amorphous or crystalline?

While some thermoplastics, like polystyrene and ABS, are categorized as amorphous, others, like HDPE and polypropylene, are widely used in the packaging sector and are classified as semi-crystalline.

What are examples of amorphous plastics?

The thermoplastics ABS, polystyrene, polycarbonate, polysulfone, and polyetherimide are examples of amorphous thermoplastics. Polyethylene, polypropylene, nylon, acetal, polyethersulfone, and polyetheretherketone are examples of crystalline plastics.

Do amorphous thermoplastics melt?

When heated, amorphous solids don't melt instantly. Rather, they reach a range of temperatures when the material changes from being glassy to rubber-like or the opposite. As a result, amorphous polymers have a glass transition temperature, or Tg, rather than a melting point.

What are the 3 types of thermoplastics?

Main types of thermoplastics
  • LDPE can be translucent or transparent. Suitable for contact with food, it is the lightest and most heat-sensitive plastic. ...
  • PVC (polyvinyl chloride) ...
  • PP (polypropylene) ...
  • PS (polystyrene or styrofoam)


Because of the way their molecular structure is set up, semi-crystalline thermoplastics are distinct from amorphous thermoplastics. Semi-crystalline polymers have properties like a well-defined melting point, better strength, improved fatigue performance, good chemical resistance, and improved wear resistance because of their ordered structure. Amorphous polymers, on the other hand, have a randomized structure that gives them better dimensional stability and a wider range of temperatures over which they soften. However, they can only be used in structural applications.


About Frank

With a background as an injection molding engineer at Kaysun, Frank brings unique depth and insight to his role as Business Development Engineer. His ability to synthesize information and pair need with opportunity is a benefit to prospects and customers alike.

Leave a Reply